1. Field of the Invention
Graft copolymers of starch-containing materials (SCM) with unsaturated organic monomers are well known in the art and can be tailored for use in many diverse applications. For example, starch graft copolymers having the appropriate ionic functionalities have been extensively used in paper and mineral separation industries as pigment retention aids capable of adjunctly serving as internal sizing agents or as flocculants. A discussion of prior art uses of such water-soluble polymers is found in "Recent Advances in Ion-Containing Polymers," M. F. Hoover and G. B. Butler, J. Poly. Sci. Symp. No. 45: 32-34 (1974). However, the future of SCM graft copolymers as an alternative to other functional agents may well hinge upon the introduction of a simple and economical procedure to prepare them. This invention relates to an improved process for the graft polymerization of acrylic monomers onto SCM.
2. Description of the Prior Art
Starch graft polymerizations are conventionally promoted by initiation of free radicals on the starch backbone by (1) chemical treatment, (2) physical treatment, or (3) irradiation. Reviews of these prior art procedures are found in Block and Graft Copolymerization, Vol. 1, Chapters 1 and 2, Ed. R. J. Ceresa, John Wiley & Sons, Inc., New York, N.Y. (1973) and "Starch, Graft Copolymers," Encyclopedia of Polymer Science and Technology, Supplement No. 2, George F. Fanta and E. B. Bagley, pp. 665-699, John Wiley & Sons, Inc., New York, N.Y. (1977).
Chemical procedures include treatment with (a) inorganic ions, e.g., ceric, chromic, and cobaltic; (b) redox systems incorporating a reducing agent and an oxidizing agent, such as ferrous ion-peroxide; and (c) organic materials, e.g., azo compounds, or solvents such as xylene, etc. All previously known free radical initiations by chemical methods have required a liquid medium which comprises either an aqueous solvent or a combination of aqueous and organic solvents. Consequently, recovery of the polymerization product involves isolation, washing, and drying steps. These steps are often the most difficult and expensive in preparation of SCM graft copolymers because high viscosities develop as the reaction progresses. Also the spent reaction medium has to be recovered and processed in order to avoid contamination of the environment. This has lead to the investigation of several dry methods for preparing SCM graft copolymers.
Physical procedures for initiating free radicals which can be conducted in the dry state include ball milling [J. Poly. Sci. 62(174): S123-S125 (1962), R. L. Whistler and J. L. Goatley], mechanical mastication [Staerke 16(9): 279-285 (1964), B. H. Thewlis], and heat and mastication as by an extruder or similar device ["Water-Soluble Polymers,"Polymer Science and Technology, Vol. 2, G. F. Fanta et al., pp. 275-290, Plenum Publishing Corp., New York, N.Y. (1973)]. The resulting products from these procedures are actually block polymers, and they tend to be highly degraded, rubbery to hard, and both chemically and physically brittle.
More useful grafted starch products, although degraded, have been prepared by a dry irradiation technique. Cobalt 60 has been used to initiate the free radicals as described in "Water-Soluble Polymers," supra, and U.S. Pat. No. 3,976,552. Other types of conventional irradiation include electron beam, ultraviolet, and X-rays. However, because of the advance technology required, expense and problems of scaleup, and hazardous nature of the reaction, irradiation techniques for initiation of free radicals in dry grafting of unsaturated organic monomers onto SCM has remained only a laboratory curiosity. These above factors all reduce the commercial desirability and practicability of the prior art methods of producing SCM acrylic graft copolymers.